HTML

Reason: HTML tends to nest pretty deeply, and understandably so. Thus, a small indentation size works well in this case.

Reason: Staying with all-lowercase for the code elements of HTML allows the actual content, which tends to be mixed-case, to stand out a little better among all of the tags and attributes. Content is king, after all.

html-attr: Don’t include whitespace before and after the = symbol in attributes.

Reason

Eliminating this whitespace helps attributes look like a more cohesive unit.

Example

  <div id="introduction" class="inset">
    1 + 1 = 3 is an interesting design principle
    pertaining to borders vs. whitespace.
  </div>

Anti-Example

  <div id = "introduction" class = "inset">
    1 + 1 = 3 is an interesting design principle
    pertaining to borders vs. whitespace.
  </div>

html-mvc: Be cognizant of deprecated HTML elements and attributes. Some of them are like that because they violate MVC.

Reason: The modern view of HTML is that it holds the content or model of a particular web application. Elements like font and center or attributes such as color pertain to the presentation or view, which today is rightly separated into CSS.
But They Still Work!: Not surprisingly, the aforementioned elements (and more) are no longer part of HTML—but you might think they are because many web browsers still “forgive” their use. Accept no mercy.